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<p>
        The header &lt;boost/core/use_default.hpp&gt; provides the type <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">use_default</span></code> which is used by other Boost
        libraries as a sentinel type in a templates to indicate defaults.
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<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Derived</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Base</span><span class="special">,</span>
   <span class="keyword">class</span> <span class="identifier">Value</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">use_default</span><span class="special">,</span>
   <span class="keyword">class</span> <span class="identifier">CategoryOrTraversal</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">use_default</span><span class="special">,</span>
   <span class="keyword">class</span> <span class="identifier">Reference</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">use_default</span><span class="special">,</span>
   <span class="keyword">class</span> <span class="identifier">Difference</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">use_default</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">iterator_adaptor</span><span class="special">;</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Value</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">node_iterator</span>
    <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">iterator_adaptor</span><span class="special">&lt;</span><span class="identifier">node_iterator</span><span class="special">&lt;</span><span class="identifier">Value</span><span class="special">&gt;,</span> <span class="identifier">Value</span><span class="special">*,</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">use_default</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">forward_traversal_tag</span><span class="special">&gt;;</span>
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<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
  <span class="keyword">struct</span> <span class="identifier">use_default</span> <span class="special">{</span> <span class="special">};</span>
<span class="special">}</span>
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